1. Sam Stello KK4VR October, 2015
Lessons from King George Amateur Radio Club 2015 Field Day Tuned Stubs Experiment

2. Ham radio receivers can be overdriven and damaged by EMI!
Problem Statement
The energy from a typical HAM radio transmitter, when another radio’s antenna is in close proximity, can couple into the second radio and cause interference even if they are widely separated in frequency.
In the extreme, the front end of the victim radio can be seriously damaged.
Ham radio receivers can be overdriven and damaged by EMI!

4. Our Field Day Interference Problem
At our FD the last two years, we had severe cross band interference
We were working HF bands, CW and Voice on 10, 15, 20, 40, and 80 meter bands
We had disabling interference between 15 and 20 mtr CW
Power used was under 100 watts (50 typical)
We decided to investigate RF filtering solutions for 20/15 mtr interference for this year’s FD
Physically separating antennas had limited effectiveness because of FD site

5. 2015 Field Day Antenna Plan parking Route 206 Pavilion
NOTE: All measurements to trees are approximate distances to base; branches will subtract from available antenna distances
Pavilion
American Legion
Building
Shed
Grill
East
South
West
North
parking
Driveway
Route 206
145 ft
cementary
Gun Monument
power line
Pit
MOXON 20 mtr
MOXON 15 mtr
Vertical 40 mtr
Inv-V 40 mtr
Inv-V mtr
Dipole mtr
Generators
2 mtr
Estimated Coax Runs:
Dipole ft Mtr 0 ft
Inv-V ft Moxon ft
Inv-V ft Moxon ft
Inv-V ft Vertical ft

7. Should We Use Bandpass Filters?
Issues:
Tuning external filters to match radio’s frequency and adjusting filter bandwidth according to band being used is difficult over wide frequency ranges
It is difficult to match input and output impedances to 50 ohms over very wide frequency ranges
Bandpass filters’ rejected signals will reflect back to the antenna and radio!
This is a difficult approach to implement

8. Solving the problem with Band Reject Filters
Band 2,3,4,5 filters
Band Reject filters effectively short the feedline on HAM bands not being used; filters present “high impedance” to band in use
Filters work for signals in both directions, ie, both transmitted and received signals are passed or rejected according to filter settings
Requires a filter for each band rejected, but we can take advantage of the limited number of HF Field Day bands

9. Commercial Filter Products
Single Band Lumped Component HF RF filters
Typically 25 db of suppression (4 + S Units)
Filters are fixed frequency, single Ham band filters
Internet user reviews very favorable
Issues:
Manual band switching required
Filters made to order; lead time can be several months
Power limitations; filters can be destroyed by too much power
Cost is approximately $120 each band; one HF 5 band set can cost $600
“homebrew designs” available, but internet reviewers warned about difficulty to build and tune; parts availability is also a concern
Many Hams who purchased commercial filters were pleased with their purchase

10. ¼ Wave Tuned Stub Alternative Approach
A RF filter can be built from common coax
A ¼ wave coax stub at the tuned frequency will invert the impedance at the opposite end of the cable.
Assume one end of a coax is open. Then the current at that end is nearly zero (less leakage); at the other end of the coax, ¼ wave away, the current must be very high.
The inverse is true for the voltage.
THEREFORE:
Since Z = E / I, the impedance at the open end must be very high and the impedance at the opposite end is very low at the tuned frequency.
The inverse happens for an shorted coax stub, that is, the impedance at the opposite end is very high at the tuned frequency.

11. Impedance Transformation in a Coax Stub with the Open Circuit End
High Z
Low Z
Z = E / I
Current at ¼ wavelength
Current at open cable end
Voltage at open cable end
Voltage at ¼ wavelength

12. Tuned Stub Filter Design
Coax “T” connector
Coax to Radio
Coax to antenna
Physical length of cable is calculated for ¼ wave as:
Length feet = (Velocity Factor x 983.6) / 4 Freq MHZ
Length is approximate; cut longer and trim to resonance
PL259
RG8 or other HF grade coax
PL259
End of cable is open circuited

13. Multiple Tuned Stub Filters Example
Here is an example of two filters used to reduce interference from two adjacent bands
(NOTE: other radios may still have to install their own filters)
40 mtr
20 mtr
80 mtr
Example from AC0C Amateur Radio article on So2R “targeted Attenuation for Adjacent Bands”

14. Two tuned stub filters at 80 and 20 meters
for a radio operating on 40 meters
HERE IS THE 20 MTR TUNED STUB
Copied from AC0C Amateur Radio article on So2R “targeted Attenuation for Adjacent Bands”

15. Two tuned stub filters at 80 and 20 meters
for a radio operating on 40 meters
HERE IS THE 80 MTR TUNED STUB
Copied from AC0C Amateur Radio article on So2R “targeted Attenuation for Adjacent Bands”

16. Our Field Day Experiment
We built three “40 mtr tuned stub” filters using “junkbox” parts and coax
Cost was 25 feet of coax, a RF “T” connector , two PL259 and one SO239 connector per radio
Cost would be approximately $50 for each radio, if built with new components
Actuals were approximately $20 for each radio using old coax, toolbox connectors plus some new connectors
Bottom Line: our interference problem was solved at much less cost than purchasing commercial filters!

17. Our Field Day Tuned Stub Filter Design
Coax “T” connector
Physical length of cable is calculated for ¼ wave as:
Length feet = (VF x 983.6) / 4 Freq MHZ
¼ wave stub of RG 8 at 40 meters would be: (0.66x983.6) / (4 x 7MHZ) =23.2 feet
When trimming, a change of approximately 4 inches in cable length corresponds to 100khz at 7 Mhz
to Radio
to Antenna
PL259
RG8 or other HF grade coax
PL259
SO239 with center pin shorted to case
Screw bulkhead connector into PL259 for “Open”
Remove connector for “short”

19. Coax Impedance Transformation
1. A ¼ wave coax at the tuned frequency will inverse the impedance present at the opposite end of the cable.
2. A ½ wave coax at the tuned frequency will have the same impedance as present on the opposite end of the cable.
3. A ¾ quarter wave coax at the tuned frequency will act like a ¼ wave coax, that is, it will inverse the impedance present at the opposite end of the cable.
4. A full wave coax at the tuned frequency will act like a half wave coax, that is, it will have the same impedance as present on the opposite end of the cable.
5. This pattern repeats beyond one wavelength

20. Impedance Transformation in a Coax Stub with the end shorted
Low Z
High Z
Low Z
High Z
Z = E / I
Voltage at ¼ wavelength
Voltage at shorted cable end
Current at shorted cable end
Current at ¼ wavelength

21. Summary Cable Length Stub Impedance at Radio and Antenna cable end
Open cable end configuration
¼ wave
High Impedance
shorted
Low Impedance
opened
1/2 wave
¾ wave
Full wave

22. Our “Poor Man” Approach to Multiple Filters
The amateur HF ham bands are harmonically related, so
A quarter wave stub on 40 mtrs is a half wave stub on 20 mtrs!
Etc, etc, etc.
Therefore,
A shorted ¼ wavelength stub on 40 meters appears as:
High Impedance on 40 and 15 mtrs
Low Impedance on 20 and 10 mtrs
An open ended ¼ wavelength stub on 40 mtrs appears as:
Low Impedance on 40 and 15 mtrs
High Impedance on 20 and 10 mtrs

23. Harmonic Relationships of HF Bands
35 ft (free space)
35 feet is:
¼ wavelength on 7.02 mhz
40m
20m
1/2 wavelength on mhz
15m
3/4 wavelength on mhz
1 wavelength on 28.1 Mhz
10m

24. Effects on HF Ham Bands of a 40 mtr ¼ Wave Tuned Stub
Shorted Stub
Open Stub 80
------ 40
pass
short 20 15 10

25. Our FD Interference Situation
15 mtr
20 mtr

26. Assumed Interference Mechanism
20 mtr transmitter
14MHz
7MHz
21MHz
28MHz
3.5MHz
14MHz
7MHz
21MHz
28MHz
3.5MHz
15 mtr receiver

27. Our First FD Attempt at Filtering
15 mtr
20 mtr
Stub end open

28. Effects of 40 mtr Tuned Stub on 20 mtr Transmitter
14MHz
7MHz
21MHz
28MHz
3.5MHz
14MHz
7MHz
21MHz
28MHz
3.5MHz
15 mtr receiver

29. Our Second Attempt at FD Filtering
15 mtr
Stub end shorted
20 mtr
Stub end open

30. Effects of 40 mtr Tuned Stub on Transmitter and Receiver
20 mtr transmitter
14MHz
7MHz
21MHz
28MHz
3.5MHz
14MHz
7MHz
21MHz
28MHz
3.5MHz
15 mtr receiver

31. Our Field Day Tuned Stub Configuration
15 mtr
Stub end shorted
The offending 20 mtr transmitter was effectively isolated
20 mtr
Stub end open
40 mtr
Stub end shorted

32. Lessons Learned (1) Tuned Stub Filters are Easily Designed
Calculations are simple
Harmonic relationships must be considered in all designs, even single band filters
For multi-band single stub designs, band chosen must be at lowest affected frequency
Tuned Stubs are Easily Built
Fast to build
Easy to tune with an antenna analyzer
Easy to test
Be careful about the quality of the coax; some old contaminated coax has very different velocity factors
Types of center insulation of coax limits loop sizes (avoid foam centers)

33. Lessons Learned (2) Tuned Stubs Require Thoughtful Hookups
Care must be taken to ensure correct configuration for band in use…don’t short out your transmitter!
Danger! There is very high voltages on the open cable ends
¼ wave stubs only work well on higher frequency bands
Our 40 mtr stubs would not work well with Interference on 80 or 160 mtr bands!

34. Why doesn’t a 40 mtr stub work well on 80?
35 feet (free space)
35 feet is
¼ wavelength on 7.02 mhz
35 feet is
1/8 wavelength on 3.51 mhz
HOWEVER: ¼ wavelength on 80 is ½ wavelength on 40!
A 40 mtr stub will not work well on 80
but
A 80 mtr ¼ wave stub will work well on 40 and higher bands!

35. Lessons Learned (3) At field day
Using a filter on each of the interfering radios made some happy and pleasantly surprised operators
Using a single filter on one radio was insufficient
Two filters on the same radio during bench testing provided 3 dB additional suppression for one coupling mechanism, but overall effect was hardly noticeable!
Filter on the second radio was used to knock out second coupling
Filter Bandwidths covered our bands of interest with 4+ S units reduction per filter
As long as we stayed on a single band, the filters did not negatively affect our operations; switching bands required minor reconfigurations

36. Summary
Our filter approach was effective for selected crossband combinations; not useful for all band interference combinations (we knew that going in)
Tuned Stubs may not be the best EMI filter solution, but were very cost effective in our application!
Band harmonic relationships in tuned stubs can work for and against you
At next FD, we may experiment with more tuned stub filters